IQ2 A. H. ENNOR AND H. ROSENBERG 
credence, from the fact that D-amino acid oxidase could not be detected in the earth- 
worm by the present authors. 
The function of t-SEP presents a different problem to which there is, as yet, no 
categorical answer. The site of biosynthesis is now clearly defined in the bird, where it 
seems to be restricted to the kidney. The results of isotopic experiments indicate that 
L-SEP, once synthesized in the kidney, is then transported via the blood stream to the 
various organs where it appears in widely varying concentrations. The compound 
appears in the highest concentration in the small intestine and this fact, together 
with its chemical structure, suggests that it may be involved in absorption mechanisms. 
Work now in progress should lead to clarification of this point and until this is com- 
pleted it would seem fruitless to hypothesize further as to its possible functions. 
The mechanisms by which L- and D-SEP are biosynthesized are obscure at present 
and while some advance has been made with L-SEP a study of D-SEP biosynthesis 
has not yet been attempted. Elucidation of the reactions involved in L-SEP synthesis 
is a desirable, if not a necessary, prerequisite before this can be initiated. The prelimi- 
nary experiments reported here point to a possible involvement of CTP in SEP bio- 
synthesis. This and the structural similarity of this compound with the phosphatides 
suggests that the mechanism of its biosynthesis may follow pathways similar to those 
involved with these compounds. Thus the final step may be envisaged as involving 
the condensation of cytidine diphosphoethanolamine and free serine, a mechanism 
compatible with what is known of the biosynthetic pathways leading to phosphatidyl- 
serine and phosphatidylethanolamine*®: 27. It should be remembered that an additional 
problem arises in the case of D-SEP in that the origin of the D-serine moiety must be 
determined. The difficulty in isolating, for experimental purposes, different organs 
in the earthworm complicates the investigation, and initial experiments with crude 
extracts and suspensions of earthworm viscera have failed to demonstrate the pre- 
sence of an amino acid racemase. D-serine, however, could also be formed by a number 
of pathways not involving racemization, and the use of preparations of isolated organs 
from giant earthworms, common in Australia, should allow a detailed investigation 
into the origin of the D-isomer. 
ACKNOWLEDGEMENT 
This work has been partially supported by a generous grant from the Rockefeller 
Foundation. 
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